The 20-Hz signals of finback whales (B a l a e n o p t e r a p h y s a l u s)

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Abstract
The 20-Hz signals of finback whales (Balaenoptera physalus) were analyzed from more than 25 years of recordings at a variety of geographic locations on near-surface hydrophones close to whales and on deep hydrophone systems. These signals were composed of 1-s pulses of sinusoidal waveform with downward sweeping frequency from approximately 23 to 18 Hz at variable source levels up to 186 dB (re: 1 μPa at 1 m), usually with slightly lower levels for the pulses at the beginning and end of sequences. These ‘‘20-Hz’’ pulses were produced in signal bouts (separated by more than 2 h) lasting as long as 32.5 h. Bouts were composed of regularly repeated pulses at intervals of 7–26 s (typically), either at one nominal pulse rate or at two alternating (doublet) pulse intervals. Signal bouts were interrupted by rests of 1–20 min at roughly 15-min intervals and by irregular gaps lasting between 20 and 120 min. The distribution of these signals throughout the year and their temporal sequence were analyzed from the continuous drum records of the Bermuda SOFAR Station. Signal bouts occurred during winter, sometimes beginning in September and ending in May. The sound sequences were never exactly replicated. Direct association of the bouts with the reproductive season for this species points to the 20-Hz signals as possible reproductive displays by finback whales.

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